The use of copper as a conductive interconnect material is favored in semiconductor devices because of the high speed that copper provides. Copper is difficult to pattern and therefore copper interconnect leads are typically formed using damascene processing technology whereby an opening is formed in a dielectric, copper is deposited over the dielectric and within the opening, then a polishing/planarization process is used to remove copper from over the dielectric, leaving the copper inlaid within the opening. The inlaid copper includes an upper surface that is essentially co-planar with the top surface of the dielectric in which the copper is disposed. A shortcoming associated with the use of copper interconnect technology is that the exposed copper surfaces are prone to oxidation. Thus, according to conventional processing technology, a passivation or other oxidation-prevention operation is carried out after the copper surface is formed by polishing. The materials conventionally used to passivate the copper surface after polishing, however, may complex with species used in the formation of further materials over the passivated copper surface, to form undesirable contaminants.
Contact may be made to the copper surface by forming a further dielectric and further optional materials over the copper surface then etching an opening through the dielectric and the further optional materials that exposes the copper surface. In one exemplary embodiment after the polishing process forms copper interconnect structures inlaid within a dielectric, a further dielectric is formed over the structure. An etch stop layer may optionally be formed between the overlying further dielectric and the polished surface. During the deposition of such an etch stop layer, organosilicate (SiOCH) contaminants may be formed at the interface between the etch stop layer and the passivated copper surface. SiC used during the initial deposition of an etch stop layer may complex with the anti-corrosion passivated surface of copper to form the organosilicate. When the optional etch stop layer is not used, contaminants may be formed during the initial stages of the further dielectric deposition. An etching operation is then carried out to etch through the further dielectric and the optional etch stop layer exposing portions of the copper surface for the purpose of providing contact to the copper surface. Applicants have noted that, regardless of the condition of the copper surface prior to the formation of the subsequent film such as the optional etch-stop film and/or the overlying dielectric, the plasma etching processes used to etch the opening that exposes the copper surface leave contaminating etch residuals and by-products that degrade the quality of the exposed copper surface and result in undesirably increased contact resistance. It would therefore be desirable to address the integrity of the copper surface after the etching operation. The etch residuals are generally polymeric in nature and difficult to remove.
The polymeric etch residuals and by-products may include fluorine, F, carbon, C, copper, Cu and other species in various combinations. Hydrogen, H2, plasmas are conventionally used in the art of semiconductor manufacturing to remove polymeric etch by-products and residuals such as those formed due to the deposition of the etch stop layer or dielectric or during the etching operation that exposes the copper surface. Hydrogen plasmas, however, are extremely sensitive to the condition of the plasma chamber and therefore run-to-run repeatability is difficult to achieve. The via resistance for the vias formed to contact the copper surface, is dependent upon the hydrogen plasma clean and is therefore also susceptible to changes in chamber conditions and therefore unstable. Often, when a hydrogen plasma is used for cleaning, a further cleaning operation must be used due to the aforementioned shortcomings.
It would therefore be desirable to provide a robust cleaning procedure that insures that the copper surfaces upon which further conductive materials are formed, are clean, and that via resistance is thereby minimized.